1. Field of the Invention
The present invention is directed to an arrangement for compensating the weight of a displaceable component, such as a component used in the examination of a subject in a medical examination apparatus.
2. Description of the Prior Art
This type of weight compensating arrangement is employed in an X-ray examination apparatus with a target device which can be rotated around an axis together with an allocated mounting plate. The target device can also be displaceable along the mounting plate. This type of target device has a large dead weight and so requires a weight compensating arrangement, so that it is not displaced by gravity given a displacement of the mounting plate from the horizontal into the vertical, for example. Such target devices have a mass of approximately 200 to 350 kg. A displacement therefore requires the application of a large physical force. For this reason, in these types of tilting-table examination devices, the counterweight of the target device is compensated with an equally large counterweight, and a motorized support is provided for the displacement. The motorized drive of the support, which is switched on and off in the handle of the target device, for example, designed to overcome only the frictional forces of the system, which is weight-balanced in all device positions, so that the manual force required of the operator for displacing the target device is very small. It is a disadvantage that the total weight of the radiography device is increased by the counterweight, and effective collision protection requires a high outlay due to the large masses to be moved, because of their mass inertia. Solutions are known for reducing the mass of the counterweight wherein the counterweight is fixedly arranged at a long lever arm of a lever system with two arms, a spring also engaging the lever arm, and the force of the weight of the target device acts on at the shorter lever arm. This enables a mass reduction of the counterweight in proportion to the length of the lever arm. Counterweight balancing according to this principle is taught in German Utility Model 17 28 886, for example. A disadvantage of this apparatus is that the weight balancing is optimal only for a specific orientation of the tilting-table examination device at which the displacement forces for the target device are also minimal. In device positions which deviate from this, i.e. given a pivoted position which deviates from this optimal position, there is not an exact weight balancing, because the weight of the target device, which depends on the sine of the tilt angle, is balanced only for one position, namely this optional position. Although the mass can be appreciably reduced in this way, problems also arise in the collision protection due to the mass inertia, and in addition, measures are required in order to suppress the control loop oscillation of the motorized drive of the support, particularly at the beginning of a displacement, such oscillation being caused particularly by the spring. Given a constant drive power of the motorized drive of the support and an uncontrolled displacement speed, the problem arises that the displacement speed of the target device unintentionally depends on the tilt angle of the tilting-table examination device.
German OS 40 41 294 teaches a weight balancing arrangement which likewise has a weight-loaded lever arrangement for compensating the weight of the target device. One lever arm is weight-loaded, while the weight of the target device acts on the other lever arm. In this device as well, the weight of the target device is only compensated in a specific position (+90.degree.). With this arrangement, it is possible to reduce the counterweight mass depending on the lever relations. For collision protection, force sensors are allocated to the lever arm, which generate a signal for deactivating the motorized drive of the support dependent on the excursion of the lever arm from the neutral position when the target device approaches an obstacle. This principle is unusable for real technical use in an examination device, however, because an excursion of the lever arm can occur given the tilting of the target device by the changing weight acting on the lever arrangement, even though the target device does not encounter an obstacle. Measures that involve great outlay are necessary to prevent the occurrence of an unintended deactivation of the motorized drive of the support.
German AS 21 04 509 teaches a support arrangement for the displacement of a target device in which the acting displacement force is detected in a handle of the target device according to magnitude and direction by high-resolution force sensors. These force sensors are biased with the weight of the total mass of the target device. Given a weight of 3500 N, great outlay is required to evaluate a displacement force that is usually in a range between 10 and 50 N, and to execute the corresponding control of the motorized drive of the support in a reproducible manner.
German PS 24 01 853 teaches a method wherein a force sensor signal is combined with a differentiating tachometer signal for purposes of achieving a control value for advancement of the target device, in order to achieve the lowest possible weight required for the displacement and to achieve a linear relation between the size of the displacement force acting at the target device and the torque of the support drive. The generated tachometer signal has a disturbing amplitude ripple which is amplified by differentiation of the signal. A smoothing of the signal by an integrator is not possible because the dynamic behavior of the support drive is very unfavorably affected by this. The cost of this known arrangement is increased by the necessity of providing a compensation winding for preventing the lever from reacting to the non-linearity of the tachometer signal. It is likewise unfavorable that the support drive is used for compensating the weight of the target device, for which purpose energy must be continuously supplied thereto.